GaN-based Enhancement-Mode High Electron Mobility Transistors With Regrown P-GaN Gate

The GaN-based high electron mobility transistors with p-GaN gate(p-GaN E-HEMTs),which is one of the optimized solutions of enhancement-mode power electronic devices,has been accepted by the academia and industry due to its excellent performance in frequency and efficiency.Research emphasis are still focused on issues of improving the threshold voltage,reducing the on-resistance,and realizing stabilized gate reliability.The p-GaN gate regrowth technique by MOCVD presents advantages in both threshold voltage and on-resistance through thinning the AlGaN barrier in gate region and thickening it in non-gate region.This dissertation focuses on realizing high performance of normally-off HEMTs with regrown p-GaN gate,and the studies include the device simulation and design,p-GaN regrowth,device fabrication,measurement and analysis.The followings are main contents:1.Simulation of the performance of p-GaN E-HEMT by Silvaco software,including the basic electric properties,breakdown characteristics,and dynamic performance.It helps the in-depth understanding of the influence factors and regulations of the device performance from physical origins.The simulation results of basic electric properties are similar to that of real devices.The analyses of electric field distribution at OFF-state give a conclusion that lateral electric field distribution can be influenced by the length of source field plate,dielectric thickness,and trap types and concentrations in GaN buffer,while the vertical electric field distribution is mainly influenced by trap types and concentrations in GaN buffer.The time-dependent characteristics of channel current,energy band structures,and carriers' distribution are extracted out during the dynamic performance simulation at modes of OFF-ON switching and OFF-ON-OFF pulse.The analyses of simulation result establish the physical foundation for fabrication of p-GaN E-HEMTs with high performance.2.Design of the structure sizes,fabrication processes and test structures for LPCVD SiNx passivated p-GaN E-HEMTs.The specific Ron,specific gate charge,and figure of Merit(FOM)are theoretically deduced with device sizes,conductors' sheet resistance,parasitic effect,and gate charges.Based on the deduction,p-GaN E-HEMTs with large output current have been designed combining the basis of simulation results and previous measured properties.Meanwhile,the fabrication processes and its monitoring methods are also carefully designed to convert the theory to practice.3.Study of the impact of p-GaN regrowth by MOCVD on the device performance.Key scientific problems of the p-GaN regrowth technique lie in the growth of AlGaN/GaN heterostructure with thick barrier layer,precise control of AlGaN thickness in gate region,reduction of interface density at p-GaN/AlGaN interface,and control of p-GaN regrowth quality.This work has succeeded in high-quality epitaxial growth of Al0.18GaN/GaN heterostructure with a 45 nm-thick AlGaN barrier layer by adjusting the Al content and other growth conditions.The AlGaN barrier thickness is precisely controlled at 15 nm through exploiting an ICP etching method with a slow etch rate of 12 nm/min.The etch induced surface damage and contamination have been studied by XPS,AFM,etc.,and through a combination of wet solution treatment and MOCVD thermal cleaning,the interface density has reduced from 1012?1013 eV-1·cm-2 to 1011?1012 eV-1·cm-2,resulting in a lowering of gate reverse leakage current by 3?4 orders of magnitudes.The ratio of Ion/Ioff has been improved from 107 to 1010,which reaches the first-class international level.4.Study of surface passivation by high-temperature LPCVD SiNx and its impacts on device performance.Through I-V and C-V measurements,high-temperature(780?)LPCVD SiNx has been confirmed with effective suppression of surface states due to high-quality of LPCVD SiNx/AlGaN interface.However,new compatibility issues of Mg deactivation and p-GaN surface damage have emerged when passivating the p-GaN E-HEMTs with LPCVD SiNx.Fortunately,these problems can be solved by wet solution treatment and thermal cleaning,which have been studied by analyzing the surface damage through XPS and I-V characteristics.Finally,LPCVD SiNx passivated E-HEMTs with regrown p-GaN gate have been successfully fabricated with a low dynamic ratio of 1.5@VDS_OFF=250 V(23 for devices with ALD Al2O3 passivation),threshold voltage of 1.7 V@IDS=10 ?A/mm,Ron of 8.5 ?·mm,Ion/Ioff of 5×1010,gate leakage current of?2 nA/mm@VGD?-200V,?10 ?A/mm@VGS=+5 V,and breakdown voltage of 715 V@IDS=100?A/mm.The comprehensive performance ranges in the international advanced level.5.Study of the gate reliability and its degradation mechanism of the p-GaN E-HEMTs.Temperature-dependent ?-? characteristics have been measured at two different structures(structures of Metal/p-GaN/AlGaN/GaN and Metal/p-GaN/Metal)with various PMA condition.It has been found that the gate leakage current at high forward bias obeys to the Flower-Nordheim tunneling mechanism.The net acceptor concentration decreases with stressing time due to defects generation in the depletion region of the metal/p-GaN Schottky junction,which is under a high electric field strength when stressed at relatively high gate-voltage.It is confirmed by the ?-? and C-V measurement and analyses.The gate failure time obeys to the Weibull distribution and the maximum gate operation voltage is 6.87 V at a failure rate of 1%for 10-years lifetime,fulfilling the products criterion.